In recent years, with the request for speeding-up of signal transmission in electronic parts, there has been a demand for circuit boards low in electric resistance of wiring between electrodes for fabricating LSI packages such as BGA and CSP. Therefore, it is extremely important to measure the electric resistance of wiring between electrodes of such a circuit board with high precision in electrical inspection of the circuit board.
In the measurement of an electric resistance of a circuit board, there have heretofore been adopted, for example, means in which probes PA and PD for current supply and probes PB and PC for voltage measurement are pressed against and brought into contact with respective 2 electrodes 91 and 92 to be inspected, which have been electrically connected to each other in a circuit board 90 to be inspected, as illustrated in FIG. 22, a current is supplied between the proves PA and PD for current supply from a power supply device 93 in this state, and a voltage signal detected by the probes PB and PC for voltage measurement at this time is processed in an electric signal processor 94, thereby calculating an electric resistance between the electrodes 91 and 92 to be inspected.
In the method described above, however, the surfaces of the electrodes 91 and 92 to be inspected may be damaged by pressing the probes because it is necessary to bring the probes PA and PD for current supply and the probes PB and PC for voltage measurement into contact with the respective electrodes 91 and 92 to be inspected by considerably high pressing force, the probes are made of a metal, and the tips thereof are pointed. Therefore, the circuit board becomes impossible to be used. Under such circumstances, the electric resistance measurement cannot be performed on all circuit boards to be provided as products, and so, so-called sampling inspection has to be performed, and after all, the yield of the products cannot be increased.
In order to solve such a problem, there have heretofore been proposed electric resistance-measuring apparatus in which a connecting member coming into contact with electrodes to be inspected is formed by a conductive elastomer.
For example, (i) Japanese Patent Application Laid-Open No. 26446/1997 discloses an electric resistance-measuring apparatus in which elastic connecting members formed of conductive rubber obtained by binding conductive particles with an elastomer are arranged at electrodes for current supply and electrodes for voltage measurement, respectively, (ii) Japanese Patent Application Laid-Open No. 2000-74965 discloses an electric resistance-measuring apparatus having a common elastic connecting member formed of an anisotropically conductive elastomer, which is provided so as to come into contact with both surfaces of an electrode for current supply and an electrode for voltage measurement electrically connected to the same electrode to be inspected, and (iii) Japanese Patent Application Laid-Open No. 2000-241485 discloses an electric resistance-measuring apparatus comprising a circuit board for inspection on the surface of which a plurality of electrodes for inspection has been formed, and an elastic connecting member formed of a conductive elastomer, which is provided on the surface of the circuit board for inspection, wherein 2 electrodes of the electrodes for inspection are selected in a state that electrodes to be inspected have been electrically connected to the plurality of electrodes for inspection through the connecting member, one of both electrodes is used as an electrode for current supply, and the other is used as an electrode for voltage measurement to measure electric resistance.
According to such electric resistance-measuring apparatus, an electrode for current supply and an electrode for voltage measurement are brought into contact with electrodes to be inspected of a circuit board to be inspected through the elastic connecting member, whereby electrical connection is achieved, so that the measurement of the electric resistance can be conducted without damaging the electrodes to be inspected.
In LSI packages for surface mounting, such as BGA and CSP on the other hand, a wire bonding method, a TAB method, a flip chip mounting method, etc. are known as methods for mounting semiconductor chips on a circuit board making up a package. Among these mounting methods, the flip chip mounting method, which the length of wiring between a semiconductor chip and a circuit board is extremely short, have advantages from the viewpoints of miniaturization of a package and speeding-up of signal transmission.
In such a flip chip mounting method, a circuit board, on a surface of which for mounting semiconductor chips, electrodes have been formed according to a pattern corresponding to an arrangement pattern of pad electrodes of the semiconductor chips, i.e., a plurality of small-sized electrodes of the order of 10 microns have been formed at a fine pitch, is used as the circuit board making up a package, and on one hand, those, on the pad electrodes of which protruding electrodes called solder ball have been formed, are used as the semiconductor chips, wherein the pad electrode of the semiconductor chip is connected to the electrode on the circuit board through the protruding electrode formed on the semiconductor chip, whereby electrical connection between the both is achieved.
There has been recently known a flip chip mounting method in which a circuit board, on a surface for mounting semiconductor chips of which protruding electrodes formed of solder have been formed, is used in place of using the semiconductor chips, on the pad electrodes of which protruding electrodes have been formed.
However, when a measurement of an electric resistance between electrodes is performed as to the circuit boards used in such flip chip mounting methods by means of the electric resistance-measuring apparatus of the above-described constructions (i) to (iii), the following problems are involved.
In the above-described electric resistance-measuring apparatus (i) and (ii), it is necessary to electrically connect both electrode for current supply and electrode for voltage measurement at the same time to each of electrodes to be inspected in a circuit board to be inspected, which is to measure electric resistance, through the elastic connecting member. Thus, in the electric resistance-measurement apparatus for measuring an electric resistance as to a circuit board to be inspected, on which small-sized electrodes to be inspected have been arranged at a high density, it is necessary to form electrodes for current supply and electrodes for voltage measurement in a state separated from one another in a region of an area equal to or smaller than a region occupied by the electrodes to be inspected, i.e., to form electrodes for current supply and electrodes for voltage measurement much smaller in size than the electrodes to be inspected in a state separated from one another at an extremely short distance, according to individual small-sized electrodes to be inspected.
However, it is extremely difficult to manage the above, and after all, it is very difficult to produce an electric resistance-measuring apparatus for measuring the electric resistance of a circuit board having small-sized electrodes at a high density.
On the other hand, in the above-described electric resistance-measuring apparatus (iii), there is no need to form electrodes for current supply and electrodes for voltage measurement corresponding to individual electrodes to be inspected, and so it is easy to produce an electric resistance-measuring apparatus for measuring the electric resistance of a circuit board to be inspected even when small-sized electrodes to be inspected are arranged at a high density in the circuit board to be inspected.
However, such an electric resistance-measuring apparatus is great in measurement error range, and so it is difficult to perform electric resistance measurement as to a circuit board low in electric resistance between electrodes with high precision.
Further, when electric resistance measurement is performed as to a circuit board to be inspected having protruding electrodes to be inspected by means of the electric resistance-measuring apparatus of the above-described constructions (i) to (iii), the following problems are involved.
In the above-described electric resistance-measuring apparatus (i) to (iii), a pressure-sensitive conductive type anisotropically conductive sheet, in which conductive particles exhibiting magnetism are contained in an elastomer in a state oriented so as to align in the thickness-wise direction thereof, is generally used as the elastic connecting member.
On the other hand, in the circuit board having the protruding electrodes, it is difficult to control the height of projection of each electrode for reasons over the production process, and so a scatter of projected heights is generally great. Accordingly, when electric resistance measurement as to the circuit board to be inspected having the protruding electrodes to be inspected is performed, that having high irregularity-absorbing ability, specifically, an anisotropically conductive elastomer having a great thickness is used as the elastic connecting member.
However, the anisotropically conductive elastomer having a great thickness is low in the sensitivity of pressure-sensitive conductivity, and so sufficient conductivity cannot be achieved in the thickness-wise direction of the anisotropically conductive elastomer unless considerably great strain is applied to the anisotropically conductive elastomer in the thickness-wise direction thereof. Therefore, it is difficult to surely achieve the required electrical connection for electrodes to be inspected having a small projected height among a great number of electrodes to be inspected in the circuit board to be inspected, and after all, the prescribed electric resistance measurement as to such a circuit board to be inspected cannot be performed with high precision.